Fullerenes are one of the four types of naturally occurring forms of carbon, besides diamond, graphite, ceraphite and diamondoids. Fullerenes are composed entirely of carbon, and the molecules can take the form of a hollow sphere or ellipsoid.
One of the most common fullerenes is C60, also referred to as Buckminsterfullerene, the structure of which is a network of hexagons and pentagons resembling a round soccer ball (Kroto, H. W. et al., “C60: Buckminsterfullerene”, 318 Nature, pp. 162-163, November 1985). Other higher fullerenes such as C70, C76, C84, and so on have also been discovered.
Since the discovery of C60, various potential applications of fullerenes have been identified, including using fullerenes as lubricants, controlled-release agent in drugs, and a component in superconductors. Other applications of fullerenes include optical devices, carbides, chemical sensors, gas separation devices, thermal insulation, diamonds, diamond thin films, hydrogen storage, and even a pigment for toner compositions (see, U.S. Pat. No. 5,188,918). However, the difficulties in the preparation, isolation and purification of fullerenes have greatly hindered commercial exploitation thereof.
A common method for the preparation of fullerenes is by burning a hydrocarbon compound. For example, in the Huffman-Krätschmer carbon arc method (Krätschmer et al, Nature, 347, 354 (1990)), one of the more common methods for generating fullerenes today, fullerenes are prepared by heating pure carbon in the form of graphite to its plasma temperature by using graphite electrodes in arc in an inert atmosphere, which creates soot from which a fullerene mixture may be separated. The resulting crude fullerene mixture consists of 65-85% of C60 and 10-30% of C70, with higher fullerenes making up the balance of the material. Such higher fullerenes from the crude fullerene mixture include all the fullerenes higher than C70, generally C72-C200 as the soluble fullerenes.
However, due to the highly similar structure, solubility, and reactivity of the fullerenes in the mixture, with the various fullerenes only being differentiated in their molecular weight, it has been difficult to separate the discrete fullerene components from the crude fullerene mixture. On the other hand, there is high economic incentive for such separation, since highly pure fullerenes can sell at more than fifty times the price of the mixture of fullerenes.
While solvent extraction followed by adsorption of pure fullerenes, could lead to isolation of some fullerenes in highly pure form, the separation of highly pure fullerenes from a fullerene mixture has remained a difficult challenge. For example, a common method of liquid chromatography using neutral alumina as column packing (Taylor et al., J. Chem. Soc., Chem. Commun. 1423 (1992)); using graphite (Vassalo et al., J. Chem. Soc., Chem. Commun. 60 (1992)); or using activated charcoal (Scrivens et al., J. Am. Chem. Soc. 114, 7919 (1992)) has been reported, but the reported methods did not provide an efficient separation/purification of fullerenes on a large scale due to inherent losses of fullerenes attributable to chromatographic techniques. In particular, inherent losses resulted from irreversible adsorption of the fullerenes onto the adsorption medium.
To avoid the drawbacks of the chromatographic method, crystallization/recrystallization methods wherein highly pure solids are formed from a solution, have been reported. Since fullerenes are generally solid products at room temperature, it would be advantageous to use crystallization to obtain solids consisting of highly pure fullerenes from a solution containing a mixture of fullerenes.
For instance, Coustel et al. have reported in the J. Chem. Soc., Chem. Commun. 1402 (1992) that C60 crystallizes during toluene soxhlet extraction of fullerenes from soot. About 40 wt % of the fullerenes in the soot could be obtained as mostly pure C60 with trace impurities of C70. The trace impurities of C70 can be removed by a second recrystallization from a toluene soxhlet. By this method, 99.99% pure C60 can be obtained. However, while high purity C60 could be obtained, the process is very inefficient.
Prakash et al. have reported in the Chemical and Engineering News, (Sep. 20, 1993, p. 32) that C60 can be purified from C70 by precipitation of an AlCl3-C60 complex from CS2 C60 of greater than 99.9% purity can be obtained by this method. However, the use of CS2 is not desirable due to its flammability and toxicity.
Zhou et al. have reported in Carbon, 32, 935 (1994) that C60 of purity higher than 99.5% and C70 of purity greater than 98% can be separated from the crude fullerene soot extract via crystallization in organic solvents such as o-xylene, carbon disulfide and 1,3-diphenylacetone. However, these crystallization separation practices were generally developed by trial and error and the underlying principle for these methods were not well explained and understood. This lack of understanding greatly hinders the scale-up of the corresponding process for mass production and separation of highly pure C60 and C70.
Atwood et al. have reported in Nature, 368, 229 (1994); and in U.S. Pat. No. 5,711,927, that both C60 and C70 can form discrete complexes with calixarenes, bowl-shaped macrocycles with hydrophobic cavities. Therefore complexation of p-But-calyx[8]arene with a mixture of the toluene extract of crude fullerene soot, followed by a series of re-crystallizations provides over 99.5% pure C60 with a substantial reduction in the cost of purifying C60 and C70. However, this method does not yield pure C70.
In addition to the above literature, Japanese Laid-open Patent Publication No. 2004-244245 discloses a method and an apparatus for continuously isolating a specific fullerene in a crystal phase. The method and apparatus include charging a solution of fullerene in a crystallization column, evaporating and removing a solvent in the upper part of the crystallization column, preferentially depositing the specific fullerene to settle in the lower part of the crystallization column, condensing the evaporated and removed solvent in a condenser, and returning part or whole of the removed solvent to the lower part of the crystallization column to cause rising flow in the crystallization column to bring the crystal phase into counter-currently contact with the solvent phase, thereby increasing the purity of the specific fullerene in the crystal phase. However, the isolation method and apparatus provided in the Japanese patent publication are only applicable when the system of C60, C70 and the solvent shows a solid solution behavior.
Although crystallization techniques such as melt crystallization and solution crystallization, and the associated unit operations have been well known in the field, the process configuration for separating a particular mixture of compounds depends on the specific behavior of the mixture (namely, the solid-liquid equilibrium phase behavior) and thus had to be developed on a case-by-case basis.
Accordingly, there is an important need to develop a process for separating highly pure fullerenes from a mixture of fullerenes by crystallization based on a solid-liquid equilibrium phase diagram of the relevant mixture of fullerenes system. In particular, it is advantageous to develop a process that can simultaneously isolate two highly pure fullerenes from a mixture of fullerenes that mainly contain the two components. Successful commercialization of such processes will also have a significant economic impact.